The invention relates to a method for regulating the quantity of circulating solids in a circulating fluidized bed reactor system as well as to an installation for implementing the method.
Fluidized bed reactor systems are known for the most varied applications, for example in the chemical industry or in energy and power plant engineering. In the latter application, fossil fuels like coal or other combustible materials such as waste or fuels derived from waste or biomass are burned or gasified in the fluidized bed of the fluidized bed reactor. To separate and recirculate the majority of the solid particles (ash and inert material) contained in the exhaust gas in the fluidized bed reactor or in the combustion chamber, the fluidized bed reactor system exhibits one or several attached cyclone separators, generally to the side of the combustion chamber. The separated solid particles are then recirculated back into the combustion chamber. The clean gas from the cyclone separator(s) is then fed into a gas flue with convection passes in which the heat contained in the exhaust gas is transferred to a working medium (water/steam) for energy generation. (See journal Kraftwerkstechnik, Springer Verlag, 1994:2, chapter 4.3.2.3.3, “Circulating fluidized bed combustion” [pages 151 to 155], Dr. eng. Karl Strauβ).
To optimize firing with a circulating fluidized bed, an effort is made to have filtration efficiency be as high as possible and low effective separating sizes in the cyclone separators. The higher the filtration efficiency and the lower the effective separating size, the greater the quantity of circulating solids, and therefore the finer their particle size. Using a suitable cyclone separator design, it is now possible to achieve circulating material with an average particle size of d50 (50% by mass of the circulating solid particles are larger than d50) of 100 μm and less, and a cyclone separation rate of up to 99.9%. The advantages of these modern cyclone separators for the process of circulating fluidized bed combustion are as follows:                good heat transfer in the fluidized bed burner;        little additional fluidized bed material or additional use of circulating material;        good limestone use;        little deterioration of the convection passes that come in contact with the circulating solid material (e.g., tube walls and tube bundles in the fluidized bed cooler);        improved complete combustion of the fuel particles.        
However, these modern optimized cyclones have a disadvantage in that when fuels are introduced, their ash contributes greatly to the quantity of circulating solids and the flow of circulating solids becomes so great that serious operational breakdowns may occur. This problem is particularly serious where fuels are used whose ash content and/or ash characteristics vary greatly, i.e., when the fuels form varying amounts of fine-grained circulating material (i.e., material that can circulate in a fluidized bed). Transiently—when using a fuel with a high ash content—there is then too much ash and therefore too much solid in circulation. The operational breakdowns that result include, for example:                a decrease in the temperature in the fluidized bed combustion chamber, resulting in exceeding the desired temperature of the working medium;        overloading the external circulating ash system, resulting, for example, in plugging of the waste pipe or recirculation lines;        overflow of the combustion chamber distribution plate as a result of too much fluid bed material in suspension, with the result, for example, that the installation breaks down because of primary fan overload.        
It is generally advantageous for operating installations with a circulating fluid bed when both the quantity and particle size of the fluid bed material can be changed. This is why apart from the possibility of drawing off fluid bed material, so-called bed ash, from the distribution plate of the combustion chamber, i.e., to reduce the quantity of solid in the combustion chamber, a second possibility for effecting particle size of the fluidized material is generally provided, such as the use of ash sifters with devices to recirculate the ash, or the addition of supplemental inert material (noncombustible solids such as sand) with a specified particle size. However, these measures are aimed at fine-grained solids, i.e., they lead to an increase in the quantity of circulating solids.
If, however, enough fine-grain circulating material is generated from the fuel, targeted removal of this partial quantity from the quantity of circulating solids is required for stable operation.
The ash removal systems generally used on the distribution plate of the combustion chamber are, however, hardly able to remove the fine-grain solid particles because these are in almost constant circulation in the fluidized bed reactor system.
A cyclone, in particular a cyclone separator or cyclone sifter is known from printed publication DE 196 30 472 A1, which provides for the extraction of particles from the circulating solid flow that are separated out in the cyclone via the clean gas line of the cyclone. This extraction of particles is implemented by a device that interrupts the flow of gas particles and that is located in the inflow region, in an inflow housing, and/or in the cylindrical housing component, whereby the interrupting device may, among other things, be a blowing device. This interrupting device causes a loosening of the solid strands that form on the internal diameter of the cyclone separator, as a result of which improved separation of solid from gas is made possible, in particular without the undesirable ultrafine particles. In the process, use is made of the fact that after the strands are dissolved into their component particles, the fine particles are drawn off from the gas flow of the cyclone that is directed inward, and clean gas is added, while the rest of the particles are spun against the wall as a result of centrifugal forces, where they form a new strand. The use of such a cyclone in a circulating fluidized bed reactor system will lead, as described above, to a reduction in the proportion of fine material in circulation, and therefore to a disadvantageous and undesirable coarsening of their particle size.
A method for operating a fluidized bed reactor system is known from printed publication EP 0 889 943 B1, wherein a partial quantity of the solids circulating in the system is diverted for the purpose of cleaning the cooling surface in the gas cooler, and is fed into the gas cooler that is downstream from the particle separator. Mechanical cleaning and removal of deposits at the cooling surfaces results from the solid particles that have been diverted and introduced into the gas cooler. However, costly pipes are required for the purpose of diverting solids, which must be designed to withstand operating temperatures of at least 800° C.; control mechanisms such as gates, servo motors, and the like are required, which also add considerably to the cost.
A further method for operating a fluidized bed reactor system is known from printed publication DE 695 04 524 T2, whereby the reactor is a CFB reactor (CFB =Circulating Fluidized Bed), which exhibits a particle separator or cyclone separator between the reactor gas outflow and the gas cooler, which normally functions in a first separation such that it does not permit solids to get into the gas cooler in quantities and sizes sufficient to clean it. In order to introduce a sufficient quantity of solid into the gas cooler, a partial quantity of the quantity of circulating solids is extracted by reducing the separation capacity of the cyclone separator. This is done by entraining a fluid flow into the vortex present in the cyclone separator, which interrupts the vortex and as a result reduces separation capacity in comparison to normal separation capacity. In this method or system, it has been shown to be disadvantageous that, on the one hand, the vortex is interrupted within the cyclone separator and, on the other hand, that a very large quantity of fluid flow (approximately 10% of the gas circulating in the system) is necessary in order to affect the vortex. Because compressed air or steam is usually used as the fluid flow, the corresponding equipment is also required, which reduces the total effectiveness of the installation. Finally, the use of such a fluid flow increases the cost of operation considerably.
A cyclone separator is known from printed publication DE 41 36 935 A1, in which the separation can be adapted to the particular conditions of operation. This is done by providing devices or nozzles in the channel for gas inflow and/or at the immersion pipe for gas outflow, by which the flow and pressure conditions in the cyclone separator can be adjusted, or by which a gaseous medium can be entrained and distributed over the circumferential cross-section of the pipe in question. In order to achieve the desired decrease in diameter using the gaseous medium in the pipes in question, a large quantity of gaseous medium is required, which disadvantageously increases operational costs. In addition, a more cost-intensive use of large nozzles and feeder lines and ring lines and the like is needed in order to entrain the gaseous medium.
All of the described fluidized bed reactor systems or cyclone separators in which a partial quantity of the circulating solids is extracted from circulation and entrained into the clean gas have the disadvantage that their construction is either very costly and therefore cost-intensive in terms of construction and operation, or that only a certain partial quantity of the solid particle spectrum is separated or extracted, leading to the undesirable concentration of another particle spectrum in the system.